In the development of fluid flow measuring apparatus instruments are sought which will assure ever greater precision. To this end there have been developed tri dimensional fluid flow sensing devices in which separate sensing means are provided for each component of the vector which would represent normally the overall fluid flow.
This type of development has been of some considerable importance in respect of environmental studies particularly concerned with air pollution. Herein has been required the use of a precise anemometer which would be capable of recording very small air velocities and as well the fluctuations both in strength and direction of these. To this end tri dimensional anemometers have already been proposed in which sensing means are separately mounted in supporting arms which are arranged in accordance with the rectangular cartesian coordinate system. The usual arrangement is to provide one arm labelled N pointing in the north direction, another arm labelled E pointing in the east direction and a third arm labelled Z pointing vertically. Each arm may carry at its end a sensing instrument such for example as a propeller or a hot wire sensor. Each sensing means thus is responsive more or less to a single component of the fluid flow (in this case air flow) intended to be measured.
The output from the three sensing means each of which will represent one component of the vector of the fluid flow, is transmitted to a suitable instrument whereby analysis may be carried out, preferably through electronic means. A complete explanation of one known system will be found in British Pat. No. 1,238,041.
It has now been found that although many of the errors which may come into consideration in measurements of this nature may be, at least partially, corrected through electronic means in the analysing apparatus, a better precision in the basic measuring process may be obtained through a different arrangement of the arms carrying the measuring means. Thus, it has been determined that in the standard cartesian arrangement as shown by the prior art the several propellers do not show a responsiveness which can be considered rigorously cosinusoidal. This failure brings about errors both as to the intensity of the air flow and as well its direction. In the great majority of applications of the known system the wind is essentially horizontal with a minor vertical component. The standard cartesian arrangement provokes a maximum error as to the horizontal wind and a considerable uncertainty as to the vertical component of the wind. The present invention provides a novel disposition of the arms carrying the sensing means and thereby seeks to avoid the difficulties of the known prior art.